Method and apparatus for measuring light intensity for imaging

ABSTRACT

A method of measuring light intensity for imaging using a light detector array comprising a plurality of light detectors arranged to generate an output corresponding to an intensity of incident light. In a first measurement mode the light detector array generates a first plurality of output signals, each generated by one group of proximate light detectors, each group comprising a light detector pair, the first plurality of output signals each corresponding to a difference between the light intensity detected by the light detectors of the group, and generating a light intensity measurement for each group from each received output signal of the first plurality of output signals. In a second measurement mode the light detector array generates a second plurality of output signals, and a light intensity measurement is generated for each light detector from the second plurality of output signals.

TECHNICAL FIELD

The present invention relates to techniques for measuring lightintensity in imaging applications.

BACKGROUND

Near-infrared spectroscopy techniques for medical imaging, in particularfor cranial imaging are known. Such techniques can be used to measureoxygenation and microvascular function and can therefore be used todetect and assess issues such as intra-cranial hematomas.

Typically, near-infrared spectroscopy scanners use an array ofhigh-powered light emitters to direct light into the cranium of asubject and then measure corresponding light detected at an array oflight detectors. Imaging information can be generated from the detectedlight which enables information about the condition of the cranial space(such as the presence and extent of a hematoma) to be determined.

Near-infrared spectroscopy scanners are typically smaller and moreportable than other medical scanning technology such as computedtomography (CT) scanners and magnetic resonance imaging (MRI) scanners.Nevertheless, in order to achieve desirable levels of imagingresolution, high-powered light sources must be used to illuminate thesubject's head to achieve the light levels necessary to generate usefulimaging information.

As a result, use of near-infrared spectroscopy tends to be restricted toplaces where the necessary equipment (suitable power supplies etc) arefound. It is generally necessary to implement near-infrared spectroscopyscanners systems in a fixed location even though it would be desirableto provide near-infrared spectroscopy imaging techniques in a moreflexible and mobile implementation.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided amethod of measuring light intensity for imaging using a light detectorarray comprising a plurality of light detectors, each light detector ofthe plurality of light detectors arranged to generate an outputcorresponding to an intensity of incident light. The method comprises,in a first measurement mode: controlling the light detector array togenerate a first plurality of output signals, each output signal of thefirst plurality of output signals generated by one of a plurality ofgroups of proximate light detectors of the light detector array, eachgroup of proximate light detectors comprising a first light detector andsecond light detector forming a light detector pair, each output signalof the first plurality of output signals corresponding to a differencebetween the light intensity detected by the light detectors of the groupof proximate light detectors, and generating a light intensitymeasurement for each group from each received output signal of the firstplurality of output signals, the method further comprising, in a secondmeasurement mode: controlling the light detector array to generate asecond plurality of output signals, each output signal of the secondplurality of output signals generated by one of the light detectors, andgenerating a light intensity measurement for each light detector fromeach received output signal of the second plurality of output signals.

Optionally, the light detectors comprise photodiodes.

Optionally, the photodiodes of the light detector array are arranged ina linear array.

Optionally, each light detector pair comprise a photodiode paircomprising a first photodiode in series with a second photodiode.

Optionally, the anode and cathode of each photodiode are connected, viaa switching matrix to a plurality of voltage lines and measurement linesto implement the first and second measurement mode.

Optionally, the linear array of light detectors comprises a plurality ofphotodiode pairs connected in series.

Optionally, a cathode of the first photodiode of each photodiode pair isconnected to an anode of the second photodiode of each pair.

Optionally, the photodiode pairs of the linear array are arranged insequentially forward and reverse polarity.

Optionally, the first measurement mode is implemented by: holding eachphotodiode pair in a reverse bias state where a first bias voltage Vbnis applied to an anode of the first photodiode of the photodiode pairand a second bias voltage Vbp is applied to a cathode of the secondphotodiode of the photodiode pair, and a measurement voltage Vm isapplied at the cathode of the first photodiode connected to the anode ofthe second photodiode, said measurement voltage a voltage level betweenthe first bias voltage and second bias voltage, and measuring an outputof each photodiode pair corresponding to a difference in the lightdetected of the photodiode pair by measuring the current output at thecathode of the first photodiode connected to the anode of the secondphotodiode.

Optionally, the second measurement mode is implemented by: applying anull voltage Vbx to the anode of the first photodiode of each pairthereby holding the first photodiode of each pair in an unbiased,non-conducting state, and applying the second bias voltage Vbp to thecathode of the second photodiode of each pair and applying themeasurement voltage Vm at the cathode of the first photodiode connectedto the anode of the second photodiode, thereby holding the secondphotodiode of each photodiode pair in a reverse bias state, andmeasuring an output of the second photodiode of each photodiode pairfrom the current output measured at the cathode of the first photodiodeconnected to the anode of the second photodiode, and, before orsubsequently applying a first bias voltage Vbn to the anode of the firstphotodiode of each pair and applying the measurement voltage Vm at thecathode of the first photodiode connected to the anode of the secondphotodiode thereby holding the first photodiode of each pair in areverse biased state, and applying a null voltage Vbx to the cathode ofthe second photodiode of each photodiode pair thereby holding the secondphotodiode of each photodiode pair in an unbiased, non-conducting state,and measuring an output of the second photodiode of each photodiode pairfrom the current output measured at the cathode of the first photodiodeconnected to the anode of the second photodiode.

Optionally, the photodiode pairs of the linear array are arranged withthe same polarity.

Optionally, the first measurement mode is implemented by: holding eachphotodiode pair in a null bias state where a zero voltage bias isapplied to the anode and cathode of each of photodiode, and measuring anoutput of each photodiode pair corresponding to a difference in thelight detected of the photodiode pair by measuring the current output atthe cathode of the first photodiode connected to the anode of the secondphotodiode.

Optionally, the first mode is implemented by: holding each photodiodepair in a reverse bias state where a sequentially increasing voltagebias is applied to the anode of each adjacent photodiode, and measuringan output of each photodiode pair corresponding to a difference in thelight detected of the photodiode pair by measuring the current output atthe cathode of the first photodiode connected to the anode of the secondphotodiode.

Optionally, the second measurement mode is implemented by: applying afirst bias voltage to the anode of first photodiode of each pair;applying the first bias voltage to the cathode of the first diode ofeach pair and the anode of the second photodiode of each pair, therebyholding the first photodiode of each pair in an unbiased, non-conductingstate, wherein the first bias voltage sequentially increases along thephotodiode array for each photodiode pair thereby holding the secondphotodiode of each pair in a reverse bias state, and measuring an outputof the second photodiode of each photodiode pair from the current outputmeasured at the cathode of the first photodiode connected to the anodeof the second photodiode, and, before or subsequently applying the firstbias voltage to the cathode of the second photodiode of each pair;applying the same bias voltage to the cathode of the first diode of eachpair and the anode of the second photodiode of each pair, therebyholding the second photodiode of each pair in an unbiased,non-conducting state, wherein the second bias voltage sequentiallyincreases along the photodiode array for each photodiode pair therebyholding the first photodiode of each pair in a reverse bias state, andmeasuring an output of the first photodiode of each photodiode pair fromthe current output measured at the cathode of the first photodiodeconnected to the anode of the second photodiode.

Optionally, the method further comprises applying the requisite voltagesto the anodes and cathodes of the photodiodes by connecting the anodesand cathodes of the photodiodes to a plurality of voltage lines, eachvoltage line held at one of the requisite voltages.

Optionally, the anodes and cathodes of the photodiodes are connectableto the requisite voltage lines via a switching matrix.

Optionally, each voltage line is connected to a programmable voltagesupply arranged to provide for each photodiode pair and for eachphotodiode a voltage level corresponding to the first bias voltage Vbnor second bias voltage Vbp, the first bias voltage Vbn and second biasvoltage Vbp determined for each photodiode pair and for each photodiodein accordance with a calibration technique.

Optionally, the calibration technique comprises: applying referenceillumination to each photodiode and each photodiode pair, determining,for operation in the first measurement mode, the first and second biasvoltages by determining first and second voltages necessary to generatea reference output current corresponding to the reference illumination,and determining for operation in the second measurement mode, first andsecond bias voltages necessary to generate a reference output currentcorresponding to the reference illumination.

Optionally, one or more of the first and second bias voltages necessaryto generate a reference output current corresponding to the referenceillumination for operation in the first measurement mode, and/or one ormore of the first and second bias voltages necessary to generate areference output current corresponding to the reference illumination foroperation in the second measurement mode are provided by theprogrammable voltage supplies by modulating between a first and secondvoltage level.

Optionally, the method further comprises generating near-infraredspectroscopy imaging data using the light intensity measurements.

In accordance with a second aspect of the invention, there is provided:a light detector array comprising a plurality of light detectors, eachlight detector of the plurality of light detectors operable to generatean output corresponding to an intensity of incident light, saidapparatus comprising means to control the plurality of light detectors,in a first measurement mode: to generate a first plurality of outputsignals, each output signal of the first plurality of output signalsgenerated by one of a plurality of groups of proximate light detectorsof the light detector array, wherein each group of proximate lightdetectors comprises a first light detector and second light detectorforming a light detector pair, each output signal of the first pluralityof output signals corresponding to a difference between the lightintensity detected by the light detectors of the group of proximatelight detectors, said apparatus further comprising a light intensitymeasurement unit arranged to generate a light intensity measurement foreach group from each received output signal of the first plurality ofoutput signals, wherein the means to control the plurality of lightdetectors is operable, in a second measurement mode: to control theplurality of light detectors to generate a second plurality of outputsignals, each output signal of the second plurality of output signalsgenerated by one of the light detectors, and the light intensitymeasurement unit is arranged to generate a light intensity measurementfor each light detector from each received output signal of the secondplurality of output signals.

Optionally, the light detectors comprise photodiodes.

Optionally, the photodiodes of the light detector array are arranged ina linear array.

Optionally, each light detector pair comprise a photodiode paircomprising a first photodiode in series with a second photodiode.

Optionally, the anode and cathode of each photodiode are connected, viaa switching matrix to a plurality of voltage lines and measurement linesto implement the first and second measurement mode.

Optionally, the linear array of light detectors comprises a plurality ofphotodiode pairs connected in series.

Optionally, a cathode of the first photodiode of each photodiode pair isconnected to an anode of the second photodiode of each pair.

Optionally, the photodiode pairs of the linear array are arranged insequentially forward and reverse polarity.

Optionally, the first measurement mode is implemented by the means tocontrol the plurality of light detectors: holding each photodiode pairin a reverse bias state where a first bias voltage Vbn is applied to ananode of the first photodiode of the photodiode pair and a second biasvoltage Vbp is applied to a cathode of the second photodiode of thephotodiode pair, and a measurement voltage Vm is applied at the cathodeof the first photodiode connected to the anode of the second photodiodesaid measurement voltage a voltage level between the first bias voltageand second bias voltage, and the light intensity measurement unit isarranged to measure an output of each photodiode pair corresponding to adifference in the light detected of the photodiode pair by measuring thecurrent output at the cathode of the first photodiode connected to theanode of the second photodiode.

Optionally, the second measurement mode is implemented by the means tocontrol the plurality of light detectors: applying a null voltage Vbx tothe anode of the first photodiode of each pair thereby holding the firstphotodiode of each pair in an unbiased, non-conducting state, andapplying the second bias voltage Vbp to the cathode of the secondphotodiode of each pair and applying the measurement voltage Vm at thecathode of the first photodiode connected to the anode of the secondphotodiode, thereby holding the second photodiode of each photodiodepair in a reverse bias state, and the light intensity measurement unitis arranged to measure an output of the second photodiode of eachphotodiode pair from the current output measured at the cathode of thefirst photodiode connected to the anode of the second photodiode, and,before or subsequently the means to control the plurality of lightdetectors: applying a first bias voltage Vbn to the anode of the firstphotodiode of each pair and applying the measurement voltage Vm at thecathode of the first photodiode connected to the anode of the secondphotodiode thereby holding the first photodiode of each pair in areverse biased state, and applying a null voltage Vbx to the cathode ofthe second photodiode of each photodiode pair thereby holding the secondphotodiode of each photodiode pair in an unbiased, non-conducting state,the light intensity measurement unit is arranged to measure an output ofthe second photodiode of each photodiode pair from the current outputmeasured at the cathode of the first photodiode connected to the anodeof the second photodiode.

Optionally, the photodiode pairs of the linear array are arranged withthe same polarity.

Optionally, the first measurement mode is implemented by the means tocontrol the plurality of light detectors: holding each photodiode pairin a null bias state where a zero voltage bias is applied to the anodeand cathode of each of photodiode, and the light intensity measurementunit is arranged to measure an output of each photodiode paircorresponding to a difference in the light detected of the photodiodepair by measuring the current output at the cathode of the firstphotodiode connected to the anode of the second photodiode.

Optionally, the first mode is by the means to control the plurality oflight detectors: holding each photodiode pair in a reverse bias statewhere a sequentially increasing voltage bias is applied to the anode ofeach adjacent photodiode, and the light intensity measurement unit isarranged to measure an output of each photodiode pair corresponding to adifference in the light detected of the photodiode pair by measuring thecurrent output at the cathode of the first photodiode connected to theanode of the second photodiode.

Optionally, the second mode is implemented by the means to control theplurality of light detectors: applying a first bias voltage to the anodeof first photodiode of each pair; applying the first bias voltage to thecathode of the first diode of each pair and the anode of the secondphotodiode of each pair, thereby holding the first photodiode of eachpair in an unbiased, non-conducting state, wherein the first biasvoltage sequentially increases along the photodiode array for eachphotodiode pair thereby holding the second photodiode of each pair in areverse bias state, and the light intensity measurement unit is arrangedto measure an output of the second photodiode of each photodiode pairfrom the current output measured at the cathode of the first photodiodeconnected to the anode of the second photodiode, and, before orsubsequently and, before or subsequently the means to control theplurality of light detectors: applying the first bias voltage to thecathode of the second photodiode of each pair; applying the same biasvoltage to the cathode of the first diode of each pair and the anode ofthe second photodiode of each pair, thereby holding the secondphotodiode of each pair in an unbiased, non-conducting state, whereinthe second bias voltage sequentially increases along the photodiodearray for each photodiode pair thereby holding the first photodiode ofeach pair in a reverse bias state, and the light intensity measurementunit is arranged to measure an output of the first photodiode of eachphotodiode pair from the current output measured at the cathode of thefirst photodiode connected to the anode of the second photodiode.

Optionally, the means to control the plurality of light detectors isoperable to apply the requisite voltages to the anodes and cathodes ofthe photodiodes by connecting the anodes and cathodes of the photodiodesto a plurality of voltage lines, each voltage line held at one of therequisite voltages.

Optionally, the means to control the plurality of light detectorscomprises a switching matrix controlled by a control unit.

According to a third aspect of the invention, there is provided anear-infrared spectroscopy system for imaging a subject's headcomprising an imaging apparatus according to the second aspect.

According to certain examples, there is provided a method of calibratingan imaging apparatus, said imaging apparatus comprising: a photodiodearray comprising a plurality of photodiodes, said apparatus comprisingmeans to control the plurality of photodiodes, in use, in a firstmeasurement mode: to generate a plurality of output currents, eachoutput current generated by one of a plurality of pairs of adjacentphotodiodes, each output current corresponding to a difference betweenthe light intensity detected by the photodiodes of the photodiode pair,wherein the first measurement mode is implemented by the means tocontrol the plurality of photodiodes: holding each photodiode pair in areverse bias state where a first bias voltage Vbn is applied to an anodeof the first photodiode of the photodiode pair and a second bias voltageVbp is applied to a cathode of the second photodiode of the photodiodepair, and a measurement voltage Vm is applied at the cathode of thefirst photodiode connected to the anode of the second photodiode saidmeasurement voltage a voltage level between the first bias voltage andsecond bias voltage, wherein the calibration technique comprises:applying reference illumination to each photodiode pair, and determiningfor each photodiode pair the first and second bias voltages bydetermining first bias voltage Vbn and second bias voltage Vbp necessaryto generate a reference output current corresponding to the referenceillumination.

Optionally, said apparatus comprises means to control the plurality ofphotodiodes, in use, in a second measurement mode: to generate a furtherplurality of output signals, each output signal generated by one of thephotodiodes, wherein the second measurement mode is implemented byapplying a null voltage Vbx to the anode of the first photodiode of eachpair thereby holding the first photodiode of each pair in an unbiased,non-conducting state, and applying the second bias voltage Vbp to thecathode of the second photodiode of each pair and applying themeasurement voltage Vm at the cathode of the first photodiode connectedto the anode of the second photodiode, thereby holding the secondphotodiode of each photodiode pair in a reverse bias state, and, beforeor subsequently applying a first bias voltage Vbn to the anode of thefirst photodiode of each pair and the measurement voltage Vm at thecathode of the first photodiode connected to the anode of the secondphotodiode thereby holding the first photodiode of each pair in areverse biased state, and applying a null voltage Vbx to the cathode ofthe second photodiode of each photodiode pair thereby holding the secondphotodiode of each photodiode pair in an unbiased, non-conducting state,wherein the calibration technique further comprises: applying referenceillumination to each photodiode pair, and for each photodiode pair:applying a null voltage to the anode of the first photodiode of thephotodiode pair and determining the second bias voltage Vbp necessary tobe applied to the cathode of the second photodiode necessary to generatea reference output from the second photodiode corresponding to thereference illumination, and before or subsequently, applying a nullvoltage to the cathode of the second photodiode of the photodiode pairand determining the first bias voltage Vbn necessary to be applied tothe anode of the first photodiode necessary to generate a referenceoutput from the first photodiode corresponding to the referenceillumination.

Optionally, determining the first bias voltage and the second biasvoltage for each photodiode pair comprises modulating between a firstvoltage level and a second voltage level of a plurality of predeterminedvoltage levels.

In accordance with embodiments of the invention, a technique is providedfor measuring light intensity in imaging applications, and in particularnear-infrared spectroscopy medical imaging applications. In accordancewith the technique a measurement mode is provided whereby the outputsignals from light detectors of an array, for example an arraycomprising a plurality of adjacent photodiodes forming light detectorpairs, are measured by comparing their “relative” outputs (that is thedifference between the measurement of incident light generated betweenadjacent light detectors) rather than their individual outputs.

Generating light intensity measurements for imaging by comparing“relative” output of adjacent light detectors allows improved resolutionimaging to be performed, particularly for near-infrared spectroscopymedical imaging applications where the difference in incident lighttends to be small from pixel to pixel. Generating imaging data based onthe difference in light intensity between light detectors rather thanthe “absolute” light intensity detected by each light detector allowsthese small differences to be more accurately detected thus improvingthe overall resolution of the system.

Furthermore, a higher density photodiode array can be more usefully usedbecause differences in detected light intensity that would otherwise belost as noise if the output of each light detector was processedindividually (due to the closer proximity of light detectors) can bemore readily detected.

The technique further includes use of a second measurement mode in whichthe output signal from each light detector are individually measured. Inthis measurement mode large differences between pixels can be moreaccurately detected. By providing this second measurement mode,composite imaging data can be generated which includes imaging datagenerated using the first measurement mode (to identify fine detailwhere light intensity varies a smaller amount from pixel to pixel) andimaging data generated using the second measurement mode (to moreaccurately represent regions where larger differences exist, forexamples edges and so on).

In accordance with certain embodiments of the invention, differentconfigurations can be used to implement the “relative” measurement modeand the “absolute” measurement mode. In certain examples, photodiodes ofthe photodiode array are arranged in series in a linear array. Incertain embodiments, the photodiodes are connected directly to eachother in series. In certain embodiments, the photodiodes are connectedvia switching elements, for example in a switching matrix. In certainexamples the photodiode array is divided into pairs of adjacentphotodiodes. In certain examples, the polarity of the photodiodes pairsalternates along the array. In certain examples changing between the“relative” and “absolute” measurement mode comprises applying suitablebias voltages to the photodiodes of the array.

Various further features and aspects of the invention are defined in theclaims.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings where likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a simplified schematic diagram of a near-infraredspectroscopy apparatus 101 for performing imaging operations on asubject in accordance with certain embodiments of the invention;

FIG. 2 provides a simplified schematic diagram of a light detection unitin accordance with certain embodiments of the invention;

FIG. 3 provides a schematic diagram depicting the concept of a“relative” current measurement mode in accordance with certainembodiments of the invention;

FIG. 4 provides a schematic diagram of a depicting a photodiode array inaccordance with certain embodiments of the invention;

FIG. 5 a provides a schematic diagram depicting configuration of aphotodiode array for implementing a “relative” measurement mode inaccordance with certain embodiments of the invention;

FIG. 5 b provides a schematic diagram depicting configuration of aphotodiode array for implementing a first “absolute” measurement mode inaccordance with certain embodiments of the invention;

FIG. 5 c provides a schematic diagram depicting configuration of aphotodiode array for implementing a second “absolute” measurement modein accordance with certain embodiments of the invention;

FIG. 5 d provides a schematic diagram of a depicting a photodiode arrayin accordance with certain embodiments of the invention;

FIG. 5 e provides a schematic diagram depicting configuration of aphotodiode array for implementing a further “relative” measurement modein accordance with certain embodiments of the invention;

FIG. 5 f provides a schematic diagram depicting a further “relative”measurement configuration of a photodiode array in accordance withcertain embodiments of the invention;

FIG. 5 g provides a schematic diagram depicting configuration of aphotodiode array for implementing a further “relative” measurement modein accordance with certain embodiments of the invention;

FIG. 5 h provides a schematic diagram depicting configuration of aphotodiode array for implementing a first “absolute” measurement mode inaccordance with certain embodiments of the invention;

FIG. 5 i provides a schematic diagram depicting configuration of aphotodiode array for implementing a first “absolute” measurement mode inaccordance with certain embodiments of the invention;

FIG. 6 provides a schematic diagram of a light detection unit inaccordance with certain embodiments of the invention;

FIG. 7 a provides a schematic diagram of a light detection unit inaccordance with certain embodiments of the invention, in which the lightdetectors of the light detector array are individually connected toinput lines of a switching matrix;

FIG. 7 b provides a schematic diagram depicting configuration of thelight detection unit shown in FIG. 7 a to provide a “relative”measurement mode;

FIGS. 7 c and 7 d provide schematic diagrams depicting configuration ofthe light detection unit shown in FIG. 7 a to provide a first and second“absolute” measurement mode:

FIG. 8 provides a schematic diagram of a switching matrix forimplementing a system as depicted in FIGS. 7 a, 7 b, 7 c , 7 d.

FIG. 9 provides a schematic diagram illustrate a calibration techniquein accordance with certain embodiments of the invention, and

FIG. 10 provides a schematic diagram of an imaging matrix in accordancewith certain embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 provides a simplified schematic diagram of a near-infraredspectroscopy apparatus 101 for performing imaging operations on asubject.

The apparatus 101 comprises a headset connected to a measurement module102. The measurement module 102 comprises a light source unit 105, alight detection unit 106. In use, the head set 103 is fitted over thehead 104 of a subject.

The light source unit 105 comprises an array of light emitters,typically an array of light emitting photodiodes (LEDs). The LEDs may beof the same type or may be of different type, for example, two or threedifferent types, producing different wavelengths of light.

It will be understood that “light” in the context of embodiments of theinvention, refers generally to electromagnetic radiation within thefrequency range typically used in near-infrared spectroscopyapplications.

The measurement module 102 further comprises a first set of lightconduits 107 referred to as “optodes”. At a first end, each optode ofthe first set of optodes 107 is optically coupled to one of the LEDs ofthe LED array of the light source unit 105. Each optode of the first setof optodes 107 extends away from the light source unit 105 andterminates at a second end in the head set 103.

The terminating ends of the optodes of the first set of optodes arepositioned so that light from the LEDs is directed into a particularregion of the head 104 of the subject.

The measurement module 102 further comprises a second set of optodes108. At a first end, each optode of the second set of optodes 108 ispositioned within the head set 103 to detect light emitted from aparticular region of the head 104 of the subject. The light detectionunit 106 comprises an array of light detectors, typically provided by anarray of photodiodes. Each optode of the second set of optodes 108extends away from the headset 103 and terminates at a connection at thelight detection unit 106 which optically couples the optode to one ofthe photodiodes.

The measurement module 102, and the imaging processing module 109 areconnected to a control module 110 which controls operation of theapparatus 101.

In use, light is directed into the head 104 of the subject from thefirst set of optodes 107 and corresponding light emitted from the head104 of the subject is transmitted via the second set of optodes 108 tothe light detection unit 106.

Typically, the output of each LED is modulated, that is, the intensityof the light output of each LED varies in accordance with a modulatedwaveform such as a sine wave. This is typically achieved by driving eachLED with a corresponding periodic signal. Typical frequencies of theoutput of the LED array are between 10 kHz to 10 MHz. Accordingly, thelight received by the light detection unit 106 is also modulated.

The light detection unit 106 measures the received light, digitises themeasurements and communicates the digitised data to the image processingmodule. Information relating to the received light is processed by theimage processing module 109. Specifically, the image processing module109 undertakes image processing operations to generate imaging datarelating to the internal state within the head 104 of the subject inaccordance with near-infrared spectroscopy imaging techniques.

FIG. 2 provides a more detailed schematic diagram of the light detectionunit 106. The light detection unit 106 comprises a photodiode array 201,measurement processing unit 202 memory unit 203 and control unit 204.

Under the control of the control unit 204, the measurement processingunit 202 is arranged to measure the various currents within thephotodiode array, digitise them, store them and then communicate thedigitised values to the image processing module 109 for imageprocessing.

Conventionally, when measuring the intensity of light incident on anarray of photodiodes, during the measurement cycle, each photodiode isheld in a reverse bias state, and the current generated by eachphotodiode when it is exposed to light is individually measured.

However, in accordance with certain embodiments of the invention, themeasurement processing unit 202 is arranged to undertake measurements intwo modes. In a first mode, the photodiode array is segmented intoadjacent photodiode pairs, and a “relative” measurement is taken foreach photodiode pair. In the “relative” mode, the difference between thecurrent flowing in the first photodiode and the current flowing in thesecond photodiode of the photodiode pair is measured.

In a second mode, an “absolute” measurement is taken for eachphotodiode. That is, a measurement is taken in accordance withconventional techniques as described above, i.e. during a measurementcycle, each individual photodiode is held in a reverse bias, state andthe current that is generated is measured.

In certain embodiments, composite images can be generated withmeasurements generated from both the “relative” mode and the “absolute”mode.

For example, the image processing module 109 generates first imagingdata using measurements generated using the “relative” measurement mode,generates second imaging data using measurements generated using the“absolute” measurement mode, and then combines the first imaging dataand second imaging data to generate composite imaging data which, forexample, includes both fine detail between pixels, and largerdifferences between pixels, for example edges.

To implement the “relative” measurement mode and the “absolute”measurement mode, as described in more detail below, different physicalphotodiode array configurations can be used.

In certain embodiments, a photodiode array comprising a plurality ofphotodiodes connected in series is provided. The photodiode array isdivided into a plurality of photodiode pairs by the arrangement of thepolarity (i.e. the “direction” in which they are connected) of thephotodiodes. The polarity of both photodiodes in each photodiode pair isthe same (thus the cathode of one of the photodiode is connected to theanode of the other photodiode of the photodiode pair). However, thepolarity of each photodiode pair with respect to the adjacent photodiodepairs alternates (thus the anode of one of the photodiodes of a givenphotodiode pair is connected to the anode of a photodiode of an adjacentphotodiode pair and the cathode of the other of the photodiodes of thegiven photodiode pair is connected to the cathode of the cathode of aphotodiode of an adjacent photodiode pair). A simplified example of thistype of photodiode array configuration is described in more detail belowwith reference to FIG. 4 .

In other embodiments, the photodiodes of the photodiode array areconnected in series and are all connected with the same polarity (thatis the cathode of each photodiode is connected to the anode of anadjacent photodiode). A simplified example of this type of photodiodearray configuration is described in more detail below with reference toFIG. 5 d.

In both these embodiments, the “relative” measurement mode and“absolute” measurement mode can be implemented by applying certainvoltages (voltage configurations) to the photodiode arrays to hold thephotodiodes in suitable states.

In further embodiments, the photodiodes of the photodiode array are notdirectly connected in series, but are instead connected via a switchingmatrix.

This concept of the operation of the first “relative” measurement modeis depicted schematically in FIG. 3 . Specifically, FIG. 3 shows anapplication of input voltages and associated output currents, whichenable the difference in light intensity incident on two adjacentphotodiodes, connected in series, to be measured.

FIG. 3 shows a photodiode pair 301 comprising a first photodiode 301 aand a second photodiode 301 b. The first photodiode 301 a and secondphotodiode 301 b are connected in series and a first voltage bias (Vbn)is applied to the anode of the first photodiode 301 a and a secondvoltage bias Vbp is applied to the cathode of the second photodiode.

A third voltage Vm is applied at a measurement node, i.e. the pointwhere the cathode of the first photodiode 301 a is connected to theanode of the second photodiode 301 b. The voltage V_(m) is typicallyhalf the supply voltage. For example, if the supply voltage is 5V, V_(m)is typically 2.5V. The voltage V_(m) is typically halfway between thevoltage Vbp and Vbn.

In this way, the photodiode pair is reverse biased (i.e. bothphotodiodes are reverse biased).

A first current i₁ flows through the first photodiode 301 a and a secondcurrent i₂ flows through the second photodiode 301 b. A measurementcurrent flows I_(m) flows at the measurement node which is thedifference between the current flowing through the first photodiode 301a and the current flowing through the second photodiode 301 b I_(m).

As described above, the light from each LED of the LED array, and thusthe light received by each photodiode received by the photodiodes variesin intensity in accordance with a periodic signal. Accordingly, thecurrent generated by each photodiode, and thus the measurement currenti_(m) is not constant and typically comprises a DC component and an ACcomponent.

FIG. 4 provides a schematic diagram depicting a photodiode array inaccordance with certain embodiments which can be adapted to operate in afirst “relative” mode and a second “absolute” mode. Specifically, FIG. 4depicts the physical configuration of a photodiode array in which thephotodiodes are connected in series but arranged in photodiode pairs ofalternating polarity.

The photodiode array 401 comprises six photodiodes (PD0, PD1, PD2, PD3,PD4, PD5) connected in series.

The photodiode array 401 is divided into 3 photodiode pairs, a firstpair 402 comprising the first and second photodiode (PD0 and PD1), asecond pair 403 comprising the third and fourth photodiode (PD2 and PD3)and a third pair 404 comprising the fifth and sixth photodiode (PD4 andPD5).

As can be seen from FIG. 4 , the cathode of the first photodiode in eachpair (PD0, PD3, and PD4) is connected to the anode of the secondphotodiode in each pair (PD1, PD2 and PD5). However, the polarity of thephotodiodes from of the second photodiode pair 403 is reversed inrelation to the polarity of the photodiodes from the first photodiodepair 402 and the third photodiode pair 404. That is, the cathode of thesecond photodiode PD1 is connected to the cathode of the thirdphotodiode PD2 and the anode of the fourth photodiode PD3 is connectedto the anode of the fifth photodiode PD4. The photodiode array 401comprises seven voltage nodes that can be controlled: a first voltagenode V0 at the anode of the first photodiode PD0; a second voltage nodeV1 at the cathode of the first photodiode PD0 and the anode of thesecond photodiode PD1; a third voltage node V2 at the cathode of thesecond photodiode PD1 and the cathode of the third photodiode PD2; afourth voltage node V3 at the anode of the third photodiode PD2 and thecathode of the fourth photodiode PD3; a fifth voltage node V4 at theanode of the fourth photodiode PD3 and the anode of the fifth photodiodePD4; a sixth voltage node V5 at the cathode of the fifth photodiode PD4and the anode of the sixth photodiode PD5, and a seventh voltage node V6at the cathode of the sixth photodiode PD5.

In accordance with certain embodiments, the voltage expressed at eachnode connected to a photodiode anode or photodiode cathode (the voltageconfiguration) can be controlled in order to change the mode of thearray so that it operates in a “relative” measurement mode where thedifference in the current flow between the photodiodes in each pair ismeasured, or an “absolute” measurement mode where the current flow inthe first photodiode (PD0), third photodiode (PD2) and fourth photodiode(PD3) photodiode is measured and a second “absolute” measurementconfiguration where the current flow in second photodiode (PD1), fourthphotodiode (PD1) and sixth photodiode (PD5) photodiode is measured.

In one example voltage configuration to implement an example of the“relative” measurement mode, the voltage levels of the voltage nodes areset as follows (where “Vm” is the measurement voltage, i.e. a node atwhich the current measurement is taken; Vbn is a first bias voltagewhich is the voltage to apply to a photodiode anode to hold it in areverse bias state “relative” to Vm; Vbp is a second bias voltage whichis the voltage to apply to a photodiode cathode to hold it in a reversebias state “relative” to Vm. Vbx is a null voltage, where a photodiodeis held in an unbiased state.

Typically, Vbn<Vm<Vbp.

Typically, Vm<Vbx<Vm+Vf (where Vf is the forward voltage of thephotodiode):

As can be seen from FIG. 5 a , V0=Vbn; V1=Vm1; V2=Vbp; V3=Vm2; V4=Vbn;V5=Vm3; and V6=Vbp.

This is shown in FIG. 5 a . Specifically, FIG. 5 a depicts a voltageconfiguration which when applied to a photodiode array of the typedescribed with reference to FIG. 4 , implements an example of the“relative” measurement mode.

As can be seen from FIG. 5 a , by virtue of this application of biasvoltages, the first photodiode PD0 and the second photodiode PD1 areheld in a reverse bias state by virtue of the third voltage node beingheld at Vbp and the first voltage node being held at Vbn.

As a result, the net current I_(m1) at the second voltage node (Vm1) isthe current flowing through the second photodiode PD1 i_(PD1) less thecurrent flowing through the first photodiode PD0 i_(PD0).

Similarly, the fourth photodiode PD3 and the third photodiode PD2 areheld in a reverse bias state by virtue of the third voltage node beingheld at Vbp and the fifth voltage node being held at Vbn. As a result,the net current I_(m2) at the fourth voltage node (Vm2) is the currentflowing through the third photodiode PD2 i_(PD2) less the currentflowing through the fourth photodiode PD3 i_(PD3).

Similarly, the fifth photodiode PD4 and the sixth photodiode PD5 areheld in a reverse bias state by virtue of the seventh voltage node beingheld at Vbp and the fifth voltage node being held at Vbn. As a result,the net current I_(m3) at the sixth voltage node (Vm3) is the currentflowing through the sixth photodiode PD5 i_(PD5) less the currentflowing through the fifth photodiode PD4 i_(PD4).

In one example, to implement the “absolute” measurement mode in thephotodiode array configuration described with reference to FIG. 4 , twovoltage configurations are used. In the first voltage configuration, theincident light detected by a first half of the photodiodes is measured,and in the second voltage configuration, the incident light detected bythe second half of the photodiodes is measured. Typically, therefore,when measuring the incident light in the “absolute” measurement mode inthis way, the first voltage configuration is used, followed by thesecond voltage configuration.

In a first of these voltage configurations, the voltage levels of thevoltage nodes are set as follows (where Vbx is a null voltage in which aphotodiode is held in an unbiased state):

V0=Vbx; V1=Vm1; V2=Vbp; V3=Vm2; V4=Vbx; V5=Vm3; and V6=Vbp.

The operation of this first voltage configuration is shown in FIG. 5 b.

As can be seen from FIG. 5 b , by virtue of this application of biasvoltages, the first photodiode PD0 is held in an unbiased state byvirtue of the application of the null voltage on the first voltage nodeand thus it can be approximated that no current is generated. However,as the third voltage node is held at Vbp, the second photodiode PD1 isheld in a reverse bias state. Therefore, the current I_(m1) at thesecond voltage node Vm1 is the current flowing through second photodiodePD1.

Similarly, the fourth photodiode PD3 is held in an unbiased state byvirtue of the application of the null voltage on the fifth voltage nodeand thus it can be approximated that no current is generated. However,as the third voltage node is held at Vbp, the third photodiode PD2 isheld in a reverse bias state. Therefore, the current I_(m2) at thefourth voltage node Vm2 is the current i_(PD2) flowing through thirdphotodiode PD2.

Similarly, the fifth photodiode PD4 is held in an unbiased state byvirtue of the application of the null voltage on the fifth voltage nodeand thus it can be approximated that no current is generated. However,as the seventh voltage node is held at Vbp, the sixth photodiode PD5 isheld in a reverse bias state. Therefore, the current I_(m3) at the sixthvoltage node Vm3 is the current i_(PD5) flowing through sixth photodiodePD5. The current flowing through the sixth photodiode PD5

In a second of these voltage configurations, the voltage levels of thevoltage nodes are set as follows:

V0=Vbn; V1=Vm1; V2=Vbx; V3=Vm2; V4=Vbn; V5=Vm3; and V6=Vbx.

The operation of this second voltage configuration is shown in FIG. 5 c.

As can be seen from FIG. 5 c , by virtue of this application of biasvoltages, the second photodiode PD1 is held in an unbiased state byvirtue of the application of the null voltage on the third voltage nodeand thus it can be approximated that no current is generated. However,as the first voltage node is held at Vbn, the first photodiode PD0 isheld in a reverse bias state. Therefore, the current I_(m1) at thesecond voltage node Vm1 is the current flowing through second photodiodePD1 (note the current flows from the second voltage node Vm1).

Similarly, the third photodiode PD2 is held in an unbiased state byvirtue of the application of the null voltage on the third voltage nodeand thus it can be approximated that no current is generated. However,as the fifth voltage node is held at Vbn, the fourth photodiode PD3 isheld in a reverse bias state. Therefore, the current I_(m2) at thefourth voltage node Vm2 is the current i_(PD3) flowing through thefourth photodiode PD3 (note the current flows from the fourth voltagenode Vm2). The current flowing through the fourth photodiode PD3

Similarly, the sixth photodiode PD5 is held in an unbiased state byvirtue of the application of the null voltage on the seventh voltagenode and thus it can be approximated that no current is generated.However, as the fifth voltage node is held at Vbn, the fifth photodiodePD4 is held in a reverse bias state. Therefore, the current I_(m3) atthe sixth voltage node Vm2 is the current i_(PD4) flowing through thefifth photodiode PD4 (note the current flows from the sixth voltage nodeVm3). The current flowing through the fifth photodiode PD4

In the examples shown in FIGS. 5 a to 5 c , the array of photodiodes isdivided into photodiode pairs and the photodiode pairs are arranged withalternating polarity. In certain embodiments, the array of photodiodesis arranged so that the photodiodes are arranged in series with the samepolarity. An example of such an arrangement in FIG. 5 d . Specifically,FIG. 5 d depicts the physical configuration of a photodiode array 501 inwhich the photodiodes are connected in series with the same polarity.

In keeping with the photodiode array 401 described with reference toFIG. 4 , the photodiode array 501 shown in FIG. 5 comprises sevenvoltage nodes that can be controlled: a first voltage node V0 at theanode of the first photodiode PD0; a second voltage node V1 at thecathode of the first photodiode PD0 and the anode of the secondphotodiode PD1; a third voltage node V2 at the cathode of the secondphotodiode PD1 and the anode of the third photodiode PD2; a fourthvoltage node V3 at the cathode of the third photodiode PD2 and the anodeof the fourth photodiode PD3; a fifth voltage node V4 at the cathode ofthe fourth photodiode PD3 and the anode of the fifth photodiode PD4; asixth voltage node V5 at the cathode of the fifth photodiode PD4 and theanode of the sixth photodiode PD5, and a seventh voltage node V6 at thecathode of the sixth photodiode PD5.

In a first example, this arrangement can be configured in a “relative”measurement mode in which a voltage configuration is applied such thateach voltage node is held at 0V. The operation of this voltageconfiguration is shown in FIG. 5 e.

In this configuration V0=0V; V1=0V; V2=0V; V3=0V; V4=0V; V5=0V; andV6=0V.

As can be seen from FIG. 5 e , by virtue of this configuration thecurrent I_(m1) at the second voltage node Vm1 is the difference betweenthe current flowing through the second photodiode I_(PD1) and the firstphotodiode I_(PD0); the current I_(m2) at the third voltage node Vm2 isthe difference between the current flowing through the third photodiodeI_(PD2) and the second photodiode I_(PD1); the current I_(m3) at thefourth voltage node Vm3 is the difference between the current flowingthrough the fourth photodiode I_(PD3) and the third photodiode I_(PD2);the current I_(m4) at the fifth voltage node Vm4 is the differencebetween the current flowing through the fifth photodiode I_(PD4) and thefourth photodiode I_(PD3), and the current I_(m5) at the sixth voltagenode Vm5 is the difference between the current flowing through the sixthphotodiode I_(PD5) and the fifth photodiode I_(PD4).

In another configuration, a voltage configuration is applied thatimplements a “relative” measurement mode in a different way. In thisvoltage configuration, each voltage node is held at a sequentiallyhigher voltage ensuring that each photodiode is held in a reverse biasedstate. The operation of this voltage configuration is shown in FIG. 5 f.

In this configuration V0=Vbias; V1=2*Vbias; V2=3*Vbias; V3=4*Vbias;V4=5*Vbias; V5=6*Vbias; and V6=7*Vbias.

By virtue of this configuration, the current I_(m1) at the secondvoltage node (held at 2*Vbias) is the difference between the currentflowing through the second photodiode I_(PD1) and the first photodiodeI_(PD0); the current I_(m2) at the third voltage node (held at 3*Vbias)is the difference between the current flowing through the thirdphotodiode I_(PD2) and the second photodiode I_(PD1); the current I_(m3)at the fourth voltage node (held at 4*Vbias) is the difference betweenthe current flowing through the fourth photodiode I_(PD3) and the thirdphotodiode I_(PD2); the current I_(m4) at the fifth voltage node (heldat 5*Vbias) is the difference between the current flowing through thefifth photodiode I_(PD4) and the fourth photodiode I_(PD3), and thecurrent I_(m5) at the sixth voltage node (held at 6*Vbias) is thedifference between the current flowing through the sixth photodiodeI_(PD5) and the fifth photodiode I_(PD4).

As will be understood, for both the configuration shown in FIG. 5 e andthe configuration shown in 5 f, “relative” measurements are provided forpairs of photodiodes formed of the first photodiode PD0 and the secondphotodiode PD1; second photodiode PD1 and the third photodiode PD2;third photodiode PD2 and the fourth photodiode PD3; fourth photodiodePD3 and the fifth photodiode PD4, and the fifth photodiode PD4 and thesixth photodiode PD5.

In certain implementations, where the number of voltage levels availableis limited, the “relative” measurement mode is provided by applying avoltage configuration in which the photodiodes can be divided into“batches”. As described below, this requires an adaptation to thephysical configuration of the photodiode array.

FIG. 5 g provides a diagram depicting the application of this voltageconfiguration where the number of available voltage levels is restrictedto four bias voltage levels and depicts the adaptation to the photodiodearray.

In this configuration, V0=Vbias; V1=2*Vbias; V2=3*Vbias; V3=4*Vbias;V4=5*Vbias; V5=Vbias; V6=2*Vbias, and V7=3*Vbias.

By virtue of this configuration, the current I_(m1) at the secondvoltage node (held at 2*Vbias) is the difference between the currentflowing through the second photodiode I_(PD1) and the first photodiodeI_(PD0); the current I_(m2) at the third voltage node (held at 3*Vbias)is the difference between the current flowing through the thirdphotodiode I_(PD2) and the second photodiode I_(PD1); the current I_(m3)at the fourth voltage node (held at 4*Vbias) is the difference betweenthe current flowing through the fourth photodiode I_(PD3) and the thirdphotodiode I_(PD2).

At the fifth voltage node, the physical arrangement of the photodiode isarray is adapted. Specifically, typically the connection between thecathode of the fourth photodiode PD3 and the anode of the fifthphotodiode PD4 is broken (for example by a switch, not shown) to protectthe fifth photodiode PD4 from large forward currents.

No measurement current is collected at the fifth voltage node.

The current I_(m5) at the seventh voltage node (held at 2*Vbias) is thedifference between the current flowing through the sixth photodiodeI_(PD5) and the fifth photodiode I_(PD4).

In keeping with the embodiments described above with reference to FIGS.5 a, 5 b and 5 c for photodiode arrays in which the polarity ofphotodiode pairs alternates, in embodiments in which the photodiodearray is arranged with all the photodiodes connected in series and withthe same polarity, to implement the “absolute” measurement mode, twovoltage configurations are used. In the first voltage configuration, theincident light detected by a first half of the photodiodes is measured,and in the second voltage configuration, the incident light detected bythe second half of the photodiodes is measured. Typically, therefore,when measuring the incident light in the “absolute” measurement mode inthis way, the first voltage configuration is used, followed by thesecond voltage configuration.

FIG. 5 h provides a diagram of the implementation of such an “absolute”measurement mode by initially applying a first voltage configuration. Inthis voltage configuration the voltage levels of the voltage nodes areset as follows:

In this voltage configuration V0=Vbias; V1=Vbias (Vm); V2=2*Vbias;V3=2*Vbias (Vm); V4=3*Vbias; V5=3*Vbias (Vm); and V6=4*bias. NoteVbias=Vbias (Vm), 2*Vbias=2*Vbias (Vm) and 3Vbias=3*Vbias (Vm). NoteVbias=Vbias (Vm), 2*Vbias=2*Vbias (Vm) and 3*Vbias=3*Vbias (Vm).

By virtue of this voltage configuration, the current I_(m1) at thesecond voltage node (Vbias (Vm)) is the current through the secondphotodiode PD1 because the first photodiode PD0 is held in the unbiasedstate, it can be approximated that no current is generated by the firstphotodiode PD0.

Further, the current I_(m2) at the fourth voltage node (2*Vbias (Vm)) isthe current through the fourth photodiode PD3 because the thirdphotodiode PD2 is held in the unbiased state, it can be approximatedthat no current is generated by the third photodiode PD2.

Similarly, the current I_(m3) at the sixth voltage node (3*Vbias (Vm))is the current through the sixth photodiode PD5, because the fifthphotodiode PD4 is held in the unbiased state, it can be approximatedthat no current is generated by the fifth photodiode PD4. FIG. 5 iprovides a diagram of the implementation of the “absolute” measurementmode by subsequently applying a second voltage configuration. In thisvoltage configuration the voltage levels of the voltage nodes are set asfollows:

V0=0V; V1=Vbias (Vm); V2=Vbias; V3=2*Vbias (Vm); V4=2*Vbias; V5=3*Vbias(Vm); and V6=3*Vbias. Note, again, Vbias=Vbias (Vm), 2*Vbias=2*Vbias(Vm) and 3*Vbias=3*Vbias (Vm).

By virtue of this configuration, the current I_(m1) at the secondvoltage node (Vm1) is the current through the first photodiode PD0,because the second photodiode PD1 is held in the unbiased state, it canbe approximated that no current is generated by the second photodiodePD1.

Further, the current I_(m2) at the fourth voltage node (Vm2) is thecurrent through the third photodiode PD2, because the fourth photodiodePD3 is held in the unbiased state, it can be approximated that nocurrent is generated by the fourth photodiode PD3.

Similarly, the current I_(m3) at the sixth voltage node (Vm3) is thecurrent through the fifth photodiode PD4, because the sixth photodiodePD5 is held in the unbiased state, it can be approximated that nocurrent is generated by the sixth photodiode PD5.

In the example voltage configurations described above in which thevoltage nodes are set so that the photodiode array is in an “absolute”measurement mode (i.e. the voltage configurations depicted withreference to FIGS. 5 b, 5 c, 5 h and 5 i ) the current flowing throughthe non-biased photodiodes—i.e. those connected (either via anode orcathode) to the null voltage “Vbx” is shown as zero. This is because inthe non-biased state they have a much greater time constant than thephotodiodes in the biased configuration and therefore can be consideredto effectively contribute a zero current. For this reason, in mostimplementations they can effectively be considered to contribute zerocurrent.

However, in such voltage configurations, photodiodes in the non-biasedstate will gives rise to a small current. This can be considered to bethe current that would normally be stimulated if the photodiode was inthe reverse bias state multiplied by an attenuation factor. Ideally, theattenuation factor is zero (and, as described above, this is theassumption made in 5 b, 5 c, 5 h and 5 i). However, typically theattenuation is non-zero. The magnitude of the attenuation factor isgoverned by the frequency of the incident light. Typical frequencies ofincident light are selected so that the attenuation factor is as closeto zero as possible. For example, the typical frequency operation rangeis between 10 kHz and 10 MHz

In certain embodiments, to implement the different measurement modes,the voltage nodes of the photodiode array can be connected to therelevant voltage levels and outputs via a switching matrix.

An example of this is depicted schematically in FIG. 6 . FIG. 6 providesa schematic diagram of a light detection unit in accordance with certainembodiments of the invention.

The example depicted in FIG. 6 shows the arrangement for a photodiodearray in which photodiode pairs are arranged with alternating polarities(corresponding with the example voltage configurations described withreference to FIGS. 5 a to 5 c ).

However, it will be understood that the arrangement also works forphotodiode arrays in which the photodiodes are arranged in series withthe same polarity as depicted, for example, in FIGS. 5 d to 5 i . Theskilled person will understand that the photodiode array 601 is simplyreplaced with a photodiode array physically configured as shown, forexample, in FIG. 5 d.

In FIG. 6 , a switching matrix 601 is connected to the photodiode array600.

In operation, under the control of a control unit 606 the switchingmatrix 601 connects the relevant voltage nodes of the photodiode array600 to suitable voltage level input lines 602 and an output 603 lineheld at V_(m).

In the example shown in FIG. 6 , the voltage input lines provide a firstvoltage line at Vbp, a second voltage line at Vbn and a third voltageline at Vbx. The switching matrix 601 can therefore implement thevoltage configurations described with reference to FIGS. 5 a, 5 b and 5c . However, it will be understood that in embodiments implementingother voltage configurations, the requisite voltages required at eachnode are provided by a corresponding number of voltage input linesproviding the requisite voltages. For example, for voltageconfigurations such as those described with reference to FIG. 5 f ,multiple voltage input lines will be provided corresponding to Vbias,2*Vbias, 3*Vbias, 4*Vbias, 5*Vbias, 6*Vbias, 7*Vbias (and so ondepending on the number of photodiodes in the photodiode array).

Further, in operation, under the control of the control unit 606 theswitching matrix 601 connects the relevant voltage node of thephotodiode array to the output 603. The output 603 is connected to acurrent measurement unit 604 which converts the output to a digitalvalue which is then stored in a memory unit 605. Together, the switchingmatrix 601 and the current measurement unit 604 form the measurementprocessing unit described with reference to FIG. 2 .

The current measurement unit 604 can be provided by any suitable meansfor measuring current. The current measurement unit may be provided byan arrangement that passes the generated current across a resistance andthe corresponding voltage that is produced is measured. Measuring avoltage in this manner means that the voltage can be sampledcontinuously during the measurement cycle. Other means of measuring thecurrent include the use of op-amps as is known in the art.

In certain examples, the current measurement unit 604 may be provided bya charge collector performing a sampling operation. A single currentmeasurement unit can used or, in other implementations, multiple currentmeasurement units may be provided allowing multiple current measurementsto be performed in parallel.

During a measurement cycle in the “relative” measurement mode, thevoltage nodes are held at the required voltage level and the currentfrom the output 603 for each photodiode pair is measured. This isachieved by the switching matrix 601 sequentially connecting therelevant voltage nodes (V1, V3 and V5 in the example shown in FIG. 6 )to the output 603 and thus the current measure unit 604. The currentmeasurement unit 604 then converts the current measurement (for exampleamount of collected charge or sampled voltage level) into a digitalvalue (using, for example, a conventional analogue to digitalconvertor). The digital value for each current measure this is thencommunicated to the memory 605 for onward communication to the imageprocessing module 109.

During a measurement cycle in the “absolute” measurement mode, thevoltage nodes are held at the required voltage level and the currentfrom the output 603 for each photodiode is measured. Similarly, this isachieved by the switching matrix 601 sequentially connecting therelevant voltage nodes to the output 603 and thus the currentmeasurement unit 604. The current measurement unit 604 then converts thecurrent measurement to a digital value for onward communication to theimage processing module 109 as described above.

The measurement processing unit can be implemented in any suitable way.In certain examples, it may be integrated onto a single silicon device.

In the embodiments described above, the measurement modes are selectedby applying the requisite voltage to the requisite voltage nodes. Thismeans that the photodiodes of the photodiode array can simply beconnected directly to one another in series which can, in certainexamples, simplify the layout of the photodiode array.

However, in certain examples, the photodiodes of the photodiode arrayare not directly connected to one another in series but are insteadconnected in series via a plurality of physical switches.

FIG. 7 a provides a schematic diagram of another embodiment in which themeasurement modes are implemented by a physical switching arrangement.

FIG. 7 a corresponds to the arrangement shown in FIG. 6 except that aphotodiode array 701 is provided in which the anode and cathode of eachindividual photodiode is connected directly to input lines of aswitching matrix 702. Under the control of a control unit 703, theswitching matrix is arranged to connect the photodiode anodes andcathodes to voltage input lines 704, and to a plurality of currentmeasurement units 705 to implement the different measurement modes.

FIG. 7 b provides a schematic diagram showing the voltage connectionsnecessary to implement a first “relative” measurement configuration inwhich the “relative” current output from the first photodiode PD0 andsecond photodiode PD1; third photodiode PD2 and fourth photodiode PD3and fifth photodiode PD4 and sixth photodiode PD5 is measured.

More specifically, a first photodiode pair is formed by the firstphotodiode PD0 and the second photodiode PD1. The first photodiode isheld in a reverse bias state by applying a first bias voltage Vbn to itsanode and applying a measurement voltage Vm1 to its cathode. The secondphotodiode is held in a reverse bias state by applying a second biasvoltage Vbp to its cathode and applying the measurement voltage Vm1 toits anode. In accordance with the embodiments described above,Vbp>Vm>Vbn. The switching matrix 702 connects the cathode of the firstphotodiode PD0, the anode of the second photodiode PD1 and the firstcurrent measurement unit and connects the anode of the first photodiodePD0 and the cathode of the second photodiode PD1 to the requisitevoltage input lines 704.

A second photodiode pair is formed by the third photodiode PD2 and thefourth photodiode PD3. The third photodiode PD2 is held in a reversebias state by applying a first bias voltage Vbn to its anode andapplying a measurement voltage Vm2 to its cathode. The fourth photodiodePD3 is held in a reverse bias state by applying a second bias voltageVbp to its cathode and applying the measurement voltage Vm2 to itsanode. The switching matrix 702 connects the cathode of the thirdphotodiode PD2, the anode of the fourth photodiode PD3 and the secondcurrent measurement unit and connects the anode of the third photodiodePD2 and the cathode of the fourth photodiode PD3 to the requisitevoltage input lines 704.

A third photodiode pair is formed by the fifth photodiode PD4 and thesixth photodiode PD5. The fifth photodiode PD4 is held in a reverse biasstate by applying a first bias voltage Vbn to its anode and applying ameasurement voltage Vm3 to its cathode. The sixth photodiode PD5 is heldin a reverse bias state by applying a second bias voltage Vbp to itscathode and applying the measurement voltage Vm3 to its anode. Theswitching matrix 702 connects the cathode of the fifth photodiode PD4,the anode of the sixth photodiode PD5 and the third current measurementunit and connects the anode of the fifth photodiode PD4 and the cathodeof the sixth photodiode PD5 to the requisite voltage input lines 704.

FIG. 7 c provides a schematic diagram showing the voltage connectionsnecessary to implement a second configuration implementing a “relative”measurement mode in which the “relative” current output from the secondphotodiode PD1 and third photodiode PD2; and fourth photodiode PD3 andfifth photodiode PD4 is measured.

This configuration corresponds to that described with reference to FIG.7 b except that the photodiode pairs are formed from the secondphotodiode PD1 and the third photodiode PD2; and the fourth photodiodePD3 and the fifth photodiode PD4.

FIG. 7 d provides a schematic diagram showing the voltage connections ofa configuration necessary to implement an “absolute” measurement mode inwhich the current output from each photodiode is measured. In thisconfiguration, the switching matrix 702 connects the first bias voltageVbn to the anode of each photodiode connects the cathode of eachphotodiode to one of the measurement units (providing the measurementvoltage).

The switching matrix 702 necessary to implement the configurations shownin FIGS. 7 b, 7 c and 7 d can be provided by any suitable switchingmatrix as is known in the art.

FIG. 8 provides a schematic diagram of a switching matrix arrangementdesigned specifically to implement the measurement configurations shownin FIGS. 7 b, 7 c and 7 d . FIG. 8 provides a schematic diagram of sucha switching matrix arrangement.

Although not shown, it will be understood that in keeping with theembodiment shown in FIG. 6 , the current measurement units depicted inFIGS. 7 a, 7 b, 7 c and 7 d are connected to a memory unit in which thedigital values for the current measurement are stored for onwardcommunication to the image processing module.

In the embodiments described above, photodiodes are controlled byapplying relevant bias voltages to their anodes and cathodes.

However, the characteristics of photodiodes typically vary and thereforein some embodiments, each photodiode pair for certain “relative”measurement modes (for example described with reference to FIG. 5 a ),and each individual photodiode for certain “absolute” measurement modes(for example described with reference to FIGS. 5 b, 5 c, 5 h, 5 i )undergo a calibration process.

During calibration, a reference light level is applied to eachphotodiode/photodiode pair (e.g. light level necessary to generate zerooutput) and bias voltages are varied until output ofphotodiode/photodiode pair generates the requisite output (i.e. anoutput corresponding to the reference light level). These “optimum”voltage bias levels are then stored and then applied to the relevantphotodiodes during the imaging process.

The specific bias voltage applied during the calibration process willdepend on the particular voltage configurations and physical photodiodearray configurations used to implement the “relative” and “absolute”measurement modes.

For example, the calibration technique can be used to determine the biasvoltages necessary to implement examples of the “relative” measurementmode. As described above, in certain embodiments the “relative”measurement mode comprises holding each photodiode pair in a reversebias state where a first bias voltage Vbn is applied to an anode of thefirst photodiode of the photodiode pair and a second bias voltage Vbp isapplied to a cathode of the second photodiode of the photodiode pair,and a measurement voltage Vm is applied at the cathode of the firstphotodiode connected to the anode of the second photodiode and themeasurement voltage is a voltage level between the first bias voltageand second bias voltage. To calibrate the photodiode pairs forimplementing such examples of the “relative” measurement mode, thecalibration technique comprises applying reference illumination to eachphotodiode pair, and determining for each photodiode pair the first andsecond bias voltages to be applied to each photodiode pair bydetermining first bias voltage Vbn and second bias voltage Vbp necessaryto generate a reference output current corresponding to the referenceillumination. These determined first and second voltages are thenstored.

Correspondingly, the calibration technique can be used to determine thebias voltages necessary to implement examples of the “absolute”measurement mode. As described above, in certain embodiments the“absolute” measurement mode comprises applying a null voltage Vbx to theanode of the first photodiode of each pair thereby holding the firstphotodiode of each pair in an unbiased, non-conducting state, andapplying a first bias voltage Vbp to the cathode of the secondphotodiode of each pair and applying the measurement voltage Vm at thecathode of the first photodiode connected to the anode of the secondphotodiode, thereby holding the second photodiode of each photodiodepair in a reverse bias state, and, before or subsequently applying asecond bias voltage Vbn to the anode of the first photodiode of eachpair and the measurement voltage Vm at the cathode of the firstphotodiode connected to the anode of the second photodiode therebyholding the first photodiode of each pair in a reverse biased state, andapplying a null voltage Vbx to the cathode of the second photodiode ofeach photodiode pair thereby holding the second photodiode of eachphotodiode pair in an unbiased, non-conducting state.

To calibrate the photodiode pairs for implementing such examples of the“absolute” measurement mode the calibration technique comprises applyingreference illumination to each photodiode pair, and for each photodiodepair: applying a null voltage to the anode of the first photodiode ofthe photodiode pair and determining the first bias voltage Vbp necessaryto be applied to the cathode of the second photodiode necessary togenerate a reference output from the second photodiode corresponding tothe reference illumination, and before or subsequently, applying a nullvoltage to the cathode of the second photodiode of the photodiode pairand determining the second bias voltage Vbn necessary to be applied tothe anode of the first photodiode necessary to generate a referenceoutput from the first photodiode corresponding to the referenceillumination. These determined first and second voltages are thenstored.

To apply the stored voltages, typically the voltage input lines (e.g.lines 602 shown in FIG. 6 and lines 704 shown in FIGS. 7 a to 7 d ) aresequentially connected to programmable voltage sources which arecontrolled by the control unit to change the supplied voltage independence on which photodiodes/photodiode pairs are currentlyconnected.

In certain embodiments, determining the first bias voltage and thesecond bias voltage for each photodiode pair in each measurement modecomprises modulating between a first voltage level and a second voltagelevel of a plurality of predetermined voltage levels. Typically, thenumber of voltage levels that can be produced by the programmablevoltage sources may be limited therefore, during the calibrationprocess, whichever of the available voltage levels results in an outputclosest to the requisite output are selected. In certain embodiments,during calibration, it can be identified that the “ideal” bias voltagelevel (i.e. that necessary to apply to the photodiode pairs/photodiodeto generate the requisite output given the input reference light level)may be between two of the available voltage levels.

In such cases, in certain embodiments, in use, the programmable voltagesources are arranged to modulate between the two voltage levels whichthe ideal bias voltage is between.

A specific example of this is described below with reference to FIG. 9 .

FIG. 9 provides a schematic diagram of part of a light detection unit.For simplicity, only one photodiode pair is shown, but it will beunderstood that typically the photodiode array comprises multiplephotodiode pairs.

The input voltage node connected to the anode of the first photodiode ofthe photodiode pair 901 is connected via the switching matrix 902 to afirst programmable voltage bias unit 903. The input voltage nodeconnected to the cathode of the second photodiode of the photodiode pair901 is connected via the switching matrix 902 to a second programmablevoltage bias unit 904. The voltage node at the cathode of the firstphotodiode and the anode of the second photodiode (where the measurementvoltage is applied) is connected to a current measure unit 905 (asdescribed with reference to FIG. 6 , the current measure unit 905 istypically connected to a memory unit, however this is omitted in FIG. 9for clarity).

As described above, in operation, to measure the difference in thecurrent through the first photodiode of the photodiode pair 901, and thecurrent through the second photodiode of the photodiode pair 901, afirst bias voltage Vbn is applied to the anode of the first photodiodeof the photodiode pair 601, and a second bias voltage Vbp is applied tothe cathode of the second photodiode of the photodiode pair. Theapplication of the measurement voltage Vm which is between the firstbias voltage Vbn and the second bias voltage Vbp reverse biases bothphotodiodes and a measurement current is generated which is thedifference between the current flowing through the first photodiode andthe current flowing through the second photodiode.

However, variances between the first and second photodiode (for examplevariances in internal resistance, capacitance and forward voltage) willmean that in the reverse bias configuration, the photodiodes are likelyto produce different amounts of current for the same light level. Theprogrammable voltage bias units 903, 904 are arranged to compensate forthese variances by altering the voltage biases to accommodate for thesedifferences.

To implement this technique, typically, each photodiode pair 901undergoes a calibration cycle before the photodiode is used.

In one exemplary calibration mode, illumination of a predeterminedintensity (a reference intensity) is directed at the photodiode pair 901and a number of pre-set voltage level combinations are sequentiallyapplied via the programmable voltage bias units 903, 904 to determinewhich voltage bias combinations results in a zero current measuremeasured at the current measure unit 905.

For example, there may be four pre-set voltage levels for Vbn (Vbn₁,Vbn₂, Vbn₃ and Vbn₄) and four pre-set voltage levels for Vbp (Vbp₁,Vbp₂, Vbp₃ and Vbp₄). During the calibration phase, each of the possiblevoltage level combinations is tried to determine which combinationresults in the required current output (e.g. zero current output). Therewould be 16 possible voltage combinations in such an example:

Vbn₁ and Vbp₁ Vbn₂ and Vbp₁ Vbn₃ and Vbp₁ Vbn₄ and Vbp₁ Vbn₁ and Vbp₂Vbn₂ and Vbp₂ Vbn₃ and Vbp₂ Vbn₄ and Vbp₂ Vbn₁ and Vbp₃ Vbn₂ and Vbp₃Vbn₃ and Vbp₃ Vbn₄ and Vbp₃ Vbn₁ and Vbp₄ Vbn₂ and Vbp₄ Vbn₃ and Vbp₄Vbn₄ and Vbp₄

In certain situations, it may be determined that one of the optimumvoltages lies between a first and second of the pre-set voltages. Insuch situations, the programmable voltage bias units 703, 704 can bearranged, during operation to modulate between the first and secondpre-set voltage.

For example, during calibration it may be determined that a combinationof a Vbn₂ and Vbp₂ gives a current of +Ic_(min) and Vbn₂ and Vbp₃ givesa current of −Ic_(min). Where Ic_(min) is the lowest current levelmeasured. In such cases, it may be determined that the optimum voltagebias for the first programmable voltage bias unit 703 is to provide avoltage bias of Vbn₂, and the second programmable voltage bias unit 704to modulate between Vbp₂ and Vbp₃.

It will be understood that a similar calibration technique can beperformed for each photodiode for “absolute” measurement modes describedabove.

As described above, in certain examples the photodiodes of thephotodiode array can be arranged in series (for example in pairs ofalternating polarity, or with the same polarity). In the examplesdescribed above, the number of photodiodes in the array is small (e.g.six photodiodes) to simplify explanation of the operation of differentconfigurations of the array. However, it will be understood that thephotodiode array in typical implementations comprises many morephotodiodes. The photodiode array can be of any suitable length forexample 50 photodiodes in an array. In certain examples, a firstphotodiode array and second photodiode array thus arranged can be usedto form a 2D imaging matrix. An example of such an arrangement is shownin FIG. 10 .

For clarity, the voltage node connections in between adjacentphotodiodes are not shown.

It will be understood that in embodiments of the invention, dataprocessing components can be implemented using any suitable electronicprocessing and signal processing means of the type well known in theart. For example, the control module, control unit and image processingmodule of the near-infrared spectroscopy apparatus and the lightdetection units, described, for example, with reference to FIGS. 1 and 2, can be implemented by one or more suitably configured microprocessorsor suitably configured integrated circuits.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive. Each feature disclosed in this specification(including any accompanying claims, abstract and drawings) may bereplaced by alternative features serving the same, equivalent or similarpurpose, unless expressly stated otherwise. Thus, unless expresslystated otherwise, each feature disclosed is one example only of ageneric series of equivalent or similar features. The invention is notrestricted to the details of the foregoing embodiment(s). The inventionextends to any novel one, or any novel combination, of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), or to any novel one, or any novel combination,of the steps of any method or process so disclosed.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity. It will be understood by those within the art that, ingeneral, terms used herein, and especially in the appended claims aregenerally intended as “open” terms (e.g., the term “including” should beinterpreted as “including but not limited to,” the term “having” shouldbe interpreted as “having at least,” the term “includes” should beinterpreted as “includes but is not limited to,” etc.). It will befurther understood by those within the art that if a specific number ofan introduced claim recitation is intended, such an intent will beexplicitly recited in the claim, and in the absence of such recitationno such intent is present. For example, as an aid to understanding, thefollowing appended claims may contain usage of the introductory phrases“at least one” and “one or more” to introduce claim recitations.However, the use of such phrases should not be construed to imply thatthe introduction of a claim recitation by the indefinite articles “a” or“an” limits any particular claim containing such introduced claimrecitation to embodiments containing only one such recitation, even whenthe same claim includes the introductory phrases “one or more” or “atleast one” and indefinite articles such as “a” or “an” (e.g., “a” and/or“an” should be interpreted to mean “at least one” or “one or more”); thesame holds true for the use of definite articles used to introduce claimrecitations. In addition, even if a specific number of an introducedclaim recitation is explicitly recited, those skilled in the art willrecognize that such recitation should be interpreted to mean at leastthe recited number (e.g., the bare recitation of “two recitations,”without other modifiers, means at least two recitations, or two or morerecitations).

It will be appreciated that various embodiments of the presentdisclosure have been described herein for purposes of illustration, andthat various modifications may be made without departing from the scopeof the present disclosure. Accordingly, the various embodimentsdisclosed herein are not intended to be limiting, with the true scopebeing indicated by the following claims.

The invention claimed is:
 1. A method of measuring light intensity forimaging using a light detector array comprising a plurality of lightdetectors, each light detector of the plurality of light detectorsarranged to generate an output corresponding to an intensity of incidentlight, said method comprising, in a first measurement mode: controllingthe light detector array to generate a first plurality of outputsignals, each output signal of the first plurality of output signalsgenerated by one of a plurality of groups of proximate light detectorsof the light detector array, each group of proximate light detectorscomprising a first light detector and second light detector forming alight detector pair, each output signal of the first plurality of outputsignals corresponding to a difference between the light intensitydetected by the light detectors of the group of proximate lightdetectors, and generating a light intensity measurement for each groupfrom each received output signal of the first plurality of outputsignals, the method further comprising, in a second measurement mode:controlling the light detector array to generate a second plurality ofoutput signals, each output signal of the second plurality of outputsignals generated by one of the light detectors, and generating a lightintensity measurement for each light detector from each received outputsignal of the second plurality of output signals.
 2. A method accordingto claim 1, wherein the light detectors comprise photodiodes.
 3. Amethod according to claim 2, wherein the photodiodes of the lightdetector array are arranged in a linear array.
 4. A method according toclaim 3, wherein each light detector pair comprise a photodiode paircomprising a first photodiode in series with a second photodiode.
 5. Amethod according to claim 4, wherein the anode and cathode of eachphotodiode are connected, via a switching matrix to a plurality ofvoltage lines and measurement lines to implement the first and secondmeasurement mode.
 6. A method according to claim 4, wherein the lineararray of light detectors comprises a plurality of photodiode pairsconnected in series.
 7. A method according to claim 6, wherein a cathodeof the first photodiode of each photodiode pair is connected to an anodeof the second photodiode of each pair.
 8. A method according to claim 7,wherein the photodiode pairs of the linear array are arranged insequentially forward and reverse polarity.
 9. A method according toclaim 8, wherein the first measurement mode is implemented by: holdingeach photodiode pair in a reverse bias state where a first bias voltageVbn is applied to an anode of the first photodiode of the photodiodepair and a second bias voltage Vbp is applied to a cathode of the secondphotodiode of the photodiode pair, and a measurement voltage Vm isapplied at the cathode of the first photodiode connected to the anode ofthe second photodiode, said measurement voltage a voltage level betweenthe first bias voltage and second bias voltage, and measuring an outputof each photodiode pair corresponding to a difference in the lightdetected of the photodiode pair by measuring the current output at thecathode of the first photodiode connected to the anode of the secondphotodiode.
 10. A method according to claim 9, wherein the secondmeasurement mode is implemented by: applying a null voltage Vbx to theanode of the first photodiode of each pair thereby holding the firstphotodiode of each pair in an unbiased, non-conducting state, andapplying the second bias voltage Vbp to the cathode of the secondphotodiode of each pair and applying the measurement voltage Vm at thecathode of the first photodiode connected to the anode of the secondphotodiode, thereby holding the second photodiode of each photodiodepair in a reverse bias state, and measuring an output of the secondphotodiode of each photodiode pair from the current output measured atthe cathode of the first photodiode connected to the anode of the secondphotodiode, and, before or subsequently applying a first bias voltageVbn to the anode of the first photodiode of each pair and applying themeasurement voltage Vm at the cathode of the first photodiode connectedto the anode of the second photodiode thereby holding the firstphotodiode of each pair in a reverse biased state, and applying a nullvoltage Vbx to the cathode of the second photodiode of each photodiodepair thereby holding the second photodiode of each photodiode pair in anunbiased, non-conducting state, and measuring an output of the secondphotodiode of each photodiode pair from the current output measured atthe cathode of the first photodiode connected to the anode of the secondphotodiode.
 11. A method according to claim 7, wherein the photodiodepairs of the linear array are arranged with the same polarity.
 12. Amethod according to claim 11, wherein the first measurement mode isimplemented by: holding each photodiode pair in a null bias state wherea zero voltage bias is applied to the anode and cathode of each ofphotodiode, and measuring an output of each photodiode paircorresponding to a difference in the light detected of the photodiodepair by measuring the current output at the cathode of the firstphotodiode connected to the anode of the second photodiode.
 13. A methodaccording to claim 11, wherein the first mode is implemented by: holdingeach photodiode pair in a reverse bias state where a sequentiallyincreasing voltage bias is applied to the anode of each adjacentphotodiode, and measuring an output of each photodiode paircorresponding to a difference in the light detected of the photodiodepair by measuring the current output at the cathode of the firstphotodiode connected to the anode of the second photodiode.
 14. A methodaccording to claim 11, wherein the second measurement mode isimplemented by: applying a first bias voltage to the anode of firstphotodiode of each pair; applying the first bias voltage to the cathodeof the first diode of each pair and the anode of the second photodiodeof each pair, thereby holding the first photodiode of each pair in anunbiased, non-conducting state, wherein the first bias voltagesequentially increases along the photodiode array for each photodiodepair thereby holding the second photodiode of each pair in a reversebias state, and measuring an output of the second photodiode of eachphotodiode pair from the current output measured at the cathode of thefirst photodiode connected to the anode of the second photodiode, and,before or subsequently applying the first bias voltage to the cathode ofthe second photodiode of each pair; applying the same bias voltage tothe cathode of the first diode of each pair and the anode of the secondphotodiode of each pair, thereby holding the second photodiode of eachpair in an unbiased, non-conducting state, wherein the second biasvoltage sequentially increases along the photodiode array for eachphotodiode pair thereby holding the first photodiode of each pair in areverse bias state, and measuring an output of the first photodiode ofeach photodiode pair from the current output measured at the cathode ofthe first photodiode connected to the anode of the second photodiode.15. A method according to claim 9, comprising applying a requisitevoltages to the anodes and cathodes of the photodiodes by connecting theanodes and cathodes of the photodiodes to a plurality of voltage lines,each voltage line held at one of the requisite voltages.
 16. A methodaccording to claim 15, wherein the anodes and cathodes of thephotodiodes are connectable to the requisite voltage lines via aswitching matrix.
 17. A method according to claim 16, wherein eachvoltage line is connected to a programmable voltage supply arranged toprovide for each photodiode pair and for each photodiode a voltage levelcorresponding to the first bias voltage Vbn or second bias voltage Vbp,the first bias voltage Vbn and second bias voltage Vbp determined foreach photodiode pair and for each photodiode in accordance with acalibration technique.
 18. A method according to claim 17, wherein thecalibration technique comprises applying reference illumination to eachphotodiode and each photodiode pair determining, for operation in thefirst measurement mode, the first and second bias voltages bydetermining first and second voltages necessary to generate a referenceoutput current corresponding to the reference illumination, anddetermining for operation in the second measurement mode, first andsecond bias voltages necessary to generate a reference output currentcorresponding to the reference illumination.
 19. A method according toclaim 17, wherein one or more of the first and second bias voltagesnecessary to generate a reference output current corresponding to thereference illumination for operation in the first measurement mode,and/or one or more of the first and second bias voltages necessary togenerate a reference output current corresponding to the referenceillumination for operation in the second measurement mode are providedby the programmable voltage supplies by modulating between a first andsecond voltage level.
 20. A method according to claim 1, furthercomprising generating near-infrared spectroscopy imaging data using thelight intensity measurements.
 21. An imaging apparatus comprising: alight detector array comprising a plurality of light detectors, eachlight detector of the plurality of light detectors operable to generatean output corresponding to an intensity of incident light, saidapparatus comprising means to control the plurality of light detectors,in a first measurement mode: to generate a first plurality of outputsignals, each output signal of the first plurality of output signalsgenerated by one of a plurality of groups of proximate light detectorsof the light detector array, wherein each group of proximate lightdetectors comprises a first light detector and second light detectorforming a light detector pair, each output signal of the first pluralityof output signals corresponding to a difference between the lightintensity detected by the light detectors of the group of proximatelight detectors, said apparatus further comprising a light intensitymeasurement unit arranged to generate a light intensity measurement foreach group from each received output signal of the first plurality ofoutput signals, wherein the means to control the plurality of lightdetectors is operable, in a second measurement mode: to control theplurality of light detectors to generate a second plurality of outputsignals, each output signal of the second plurality of output signalsgenerated by one of the light detectors, and the light intensitymeasurement unit is arranged to generate a light intensity measurementfor each light detector from each received output signal of the secondplurality of output signals.
 22. An imaging apparatus according to claim21, wherein the light detectors comprise photodiodes.
 23. An imagingapparatus according to claim 22, wherein the photodiodes of the lightdetector array are arranged in a linear array.
 24. An imaging apparatusaccording to claim 23, wherein each light detector pair comprise aphotodiode pair comprising a first photodiode in series with a secondphotodiode.
 25. An imaging apparatus according to claim 24, wherein theanode and cathode of each photodiode are connected, via a switchingmatrix to a plurality of voltage lines and measurement lines toimplement the first and second measurement mode.
 26. An imagingapparatus according to claim 25, wherein the linear array of lightdetectors comprises a plurality of photodiode pairs connected in series.27. An imaging apparatus according to claim 26, wherein a cathode of thefirst photodiode of each photodiode pair is connected to an anode of thesecond photodiode of each pair.
 28. An imaging apparatus according toclaim 27, wherein the photodiode pairs of the linear array are arrangedin sequentially forward and reverse polarity.
 29. An imaging apparatusaccording to claim 28, wherein the first measurement mode is implementedby the means to control the plurality of light detectors: holding eachphotodiode pair in a reverse bias state where a first bias voltage Vbnis applied to an anode of the first photodiode of the photodiode pairand a second bias voltage Vbp is applied to a cathode of the secondphotodiode of the photodiode pair, and a measurement voltage Vm isapplied at the cathode of the first photodiode connected to the anode ofthe second photodiode said measurement voltage a voltage level betweenthe first bias voltage and second bias voltage, and the light intensitymeasurement unit is arranged to measure an output of each photodiodepair corresponding to a difference in the light detected of thephotodiode pair by measuring the current output at the cathode of thefirst photodiode connected to the anode of the second photodiode.
 30. Animaging apparatus according to claim 29, wherein the second measurementmode is implemented by the means to control the plurality of lightdetectors: applying a null voltage Vbx to the anode of the firstphotodiode of each pair thereby holding the first photodiode of eachpair in an unbiased, non-conducting state, and applying the second biasvoltage Vbp to the cathode of the second photodiode of each pair andapplying the measurement voltage Vm at the cathode of the firstphotodiode connected to the anode of the second photodiode, therebyholding the second photodiode of each photodiode pair in a reverse biasstate, and the light intensity measurement unit is arranged to measurean output of the second photodiode of each photodiode pair from thecurrent output measured at the cathode of the first photodiode connectedto the anode of the second photodiode, and, before or subsequently themeans to control the plurality of light detectors: applying a first biasvoltage Vbn to the anode of the first photodiode of each pair andapplying the measurement voltage Vm at the cathode of the firstphotodiode connected to the anode of the second photodiode therebyholding the first photodiode of each pair in a reverse biased state, andapplying a null voltage Vbx to the cathode of the second photodiode ofeach photodiode pair thereby holding the second photodiode of eachphotodiode pair in an unbiased, non-conducting state, the lightintensity measurement unit is arranged to measure an output of thesecond photodiode of each photodiode pair from the current outputmeasured at the cathode of the first photodiode connected to the anodeof the second photodiode.
 31. An imaging apparatus according to claim27, wherein the photodiode pairs of the linear array are arranged withthe same polarity.
 32. An imaging apparatus according to claim 31,wherein the first measurement mode is implemented by the means tocontrol the plurality of light detectors: holding each photodiode pairin a null bias state where a zero voltage bias is applied to the anodeand cathode of each of photodiode, and the light intensity measurementunit is arranged to measure an output of each photodiode paircorresponding to a difference in the light detected of the photodiodepair by measuring the current output at the cathode of the firstphotodiode connected to the anode of the second photodiode.
 33. Animaging apparatus according to claim 31, wherein the first mode is bythe means to control the plurality of light detectors: holding eachphotodiode pair in a reverse bias state where a sequentially increasingvoltage bias is applied to the anode of each adjacent photodiode, andthe light intensity measurement unit is arranged to measure an output ofeach photodiode pair corresponding to a difference in the light detectedof the photodiode pair by measuring the current output at the cathode ofthe first photodiode connected to the anode of the second photodiode.34. An imaging apparatus according to claim 31, wherein the second modeis implemented by the means to control the plurality of light detectors:applying a first bias voltage to the anode of first photodiode of eachpair; applying the first bias voltage to the cathode of the first diodeof each pair and the anode of the second photodiode of each pair,thereby holding the first photodiode of each pair in an unbiased,non-conducting state, wherein the first bias voltage sequentiallyincreases along the photodiode array for each photodiode pair therebyholding the second photodiode of each pair in a reverse bias state, andthe light intensity measurement unit is arranged to measure an output ofthe second photodiode of each photodiode pair from the current outputmeasured at the cathode of the first photodiode connected to the anodeof the second photodiode, and, before or subsequently the means tocontrol the plurality of light detectors: applying the first biasvoltage to the cathode of the second photodiode of each pair; applyingthe same bias voltage to the cathode of the first diode of each pair andthe anode of the second photodiode of each pair, thereby holding thesecond photodiode of each pair in an unbiased, non-conducting state,wherein the second bias voltage sequentially increases along thephotodiode array for each photodiode pair thereby holding the firstphotodiode of each pair in a reverse bias state, and the light intensitymeasurement unit is arranged to measure an output of the firstphotodiode of each photodiode pair from the current output measured atthe cathode of the first photodiode connected to the anode of the secondphotodiode.
 35. An imaging apparatus according to claim 29, wherein themeans to control the plurality of light detectors is operable to applythe requisite voltages to the anodes and cathodes of the photodiodes byconnecting the anodes and cathodes of the photodiodes to a plurality ofvoltage lines, each voltage line held at one of the requisite voltages.36. An imaging apparatus according to claim 35, wherein the means tocontrol the plurality of light detectors comprises a switching matrixcontrolled by a control unit.
 37. A near-infrared spectroscopy systemfor imaging a subject's head comprising an imaging apparatus accordingto claim 21.